The present invention concerns a display assembly including two superposed display devices and, more particularly, an assembly of this type including means allowing one of the display devices to appear to an observer selectively, to the exclusion of the other.
The present invention also concerns a timepiece including a display assembly of the aforementioned type and, more particularly, such a display assembly wherein an analogue display device is combined with a digital display device.
A display assembly including two superposed display devices is already known from European Patent No. EP 0 926 574 in the name of the Applicant. This display assembly is shown in FIG. 1 annexed to the present Patent Application.
FIG. 1 shows a timepiece of the wristwatch type designated as a whole by the general numerical reference 1. This watch 1 includes, in a conventional manner, a case 2 in the back cover 4 of which are arranged an electronic clockwork movement 6 and a battery 8 which rest on back cover 4 via a contact spring 10. Movement 6 includes electronic timekeeping circuits associated, via a control circuit, to a drive device (not shown) for a second hand 12, a minute hand 14 and an hour hand 16 which move above a dial 18 which bears hour symbols (not shown). Case 2 is also closed in a conventional manner by a crystal 20 covering all of dial 18.
Watch 1 further includes a display assembly including two superposed display devices respectively top device 22 and bottom device 24.
In the example shown in FIG. 1, bottom display device 24 includes analogue time display means formed by hands 12, 14 and 16 and dial 18.
According to the invention disclosed in European Patent No. EP 0 926 574, top display device 22 includes a double structure formed of a display cell 26 arranged above an optical valve 28, this display device 22 extending between bottom display device 24 and crystal 20.
Top display device 22 is arranged, on the one hand, so that display cell 26, arranged between crystal 20 and optical valve 28, like the latter, is transparent in a first switching state of cell 26 and valve 28, so as to make the data displayed by bottom display device 24, i.e. hands 12, 14 and 16 and dial 18, visible.
On the other hand, top display device 22 is arranged so that display cell 22 displays an item of data, for example of the alphanumerical type, and so that valve 28 masks bottom display device 24 in a second switching state.
As a result of these features, it is possible to use selectively either bottom display device 24, or top display device 22. If one chooses to use bottom display device 24, top display device 22 can be switched into its first state in which it is transparent, so that the data displayed by bottom display device 24 is visible. If, conversely, one chooses to use top display device 22, optical valve 28 is then switched into the second state in which it is opaque and reflective to completely mask bottom display device 24, while cell 26 of top display device 22 can display the desired data.
According to the particular embodiment shown in FIG. 1, cell 26 is a liquid crystal type display cell. Cell 26 includes a transparent front substrate 30, a back substrate 32 which is also transparent and a sealing frame 34 forming spacing and closing means which, with substrates 30 and 32, delimit a closed cavity in which is located a layer of liquid crystals. The opposite faces of substrates 30 and 32 include transparent electrodes respectively 36 and 38, made, for example, of indium/tin oxide. In the example illustrated, front substrate 30 carries electrodes configured in digits each formed of segments allowing alphanumerical characters to be displayed, while back substrate 32 carries an electrode extending over its entire surface.
When a voltage is applied or removed across electrode 38 and certain of electrodes 36, the liquid crystals located between these electrodes 36 and 38 are switched alternately from an absorbent state to a transparent state or vice versa, according to the type of liquid crystals in question and/or the presence and arrangement of polarisers associated with the cell. It is thus possible to display data in a light colour on a dark background or in a dark colour on a light background.
In the following description, elements which are identical to those previously described will be designated by the same numerical references.
In the example shown in FIGS. 2A and 2B annexed to the present patent Application, display cell 26 is a liquid crystal cell of the twisted nematic type of which the respective outer surfaces of substrates 30 and 32 are provided with absorbent polarisers 40 and 42 of crossed orientation. Thus, cell 26 is completely transparent in the first switching state (FIG. 2A), when no voltage is applied across the terminals of its electrodes 36, 38 (non-switched state), and is absorbent or diffusing in the second switching state (FIG. 2B), when a voltage is applied across the terminals of its electrodes 36, 38 (switched state). In the example shown, the optical valve includes, starting from the side of dial 18, a reflective polariser 44 and a liquid crystal cell of the twisted nematic type. Valve 28 is thus transparent in the non-switched state (FIG. 2A) and opaque and reflective in the switched state (FIG. 2B).
In the first switching state shown in FIG. 2A the natural non-polarised light, designated by the numerical reference 46, is vertically polarised by first absorbent polariser 40. The direction of polarisation of the light, designated by the reference 48, is then rotated by 90xc2x0 during its passage through display cell 26, then it is transmitted unmodified by second absorbent polariser 42. Polarised light 48 is then again rotated through 90xc2x0 during its passage through optical valve 28, then it is transmitted unmodified by reflective polariser 44 to dial 18. The light follows the same path during its return travel, so that the dial is visible to an observer 50.
Conversely, when cell 26 and valve 28 are in the switched state, the dial is totally masked. Indeed, as is seen in FIG. 2B, the vertically polarised light which passes through cell 26 in the switched zones of the latter is not modified, so that it is absorbed by absorbent polariser 40. In the same time, the light which passes through cell 26 outside the switched zones of the cell is rotated through 90xc2x0 as already explained above, then transmitted without being modified by absorbent polariser 42 up to valve 28. Valve 28 being in the switched state, the polariser light passes through it without being modified is totally reflected by reflective polariser 44. Dial 18 is thus masked, and the data is displayed in a dark colour on a light background with a mirror-like appearance.
The embodiment described above has several drawbacks.
The mirror-like appearance of the background on which the data displayed by display cell 26 appears is dazzling and thus makes it difficult to read this data, and this is all the more so when the ambient lighting is strong.
Moreover, since the light is reflected on reflective polariser 44, and thus at a certain distance from display cell 26, typically of the order of two to three millimetres, the images of the switched segments of cell 26 appear in projection on said reflective polariser 44, which leads to double vision of the displayed data. This is detrimental not only to the aesthetic appearance of the watch, but also, of course, to the legibility of the displayed data.
Finally, when one looks at the watch from certain angles, bottom display device 24 remains visible although top display device 22 is in the switched state in which it should normally completely mask said bottom device 24.
The object of the present invention is to overcome the aforementioned drawbacks of the prior art by providing a display assembly including two superposed, respectively top and bottom, display devices, wherein it is possible to make data displayed by one of the display devices visible to an observer selectively, to the exclusion of the data displayed by the other display device, and with improved legibility of the data displayed by the display device in question.
Another object of the present invention is to provide a display assembly wherein the top display device is a liquid crystal device with excellent display contrast.
Another object of the present invention is to provide a timepiece fitted with such a display assembly having an improved aesthetic appearance.
The present invention therefore concerns a display assembly including two superposed, respectively top and bottom, display devices, the top display device including a double structure formed of a display cell arranged above an optical valve, said double structure being arranged so that said cell and said valve are transparent in a first state to make the bottom display device visible, and so that the cell displays an item of data and so that the valve is opaque and at least partially masks the bottom display device in a second state, control means supplying a control voltage to cause the display cell and the optical valve to switch from the first state to the second state and vice versa, characterised in that the display cell is of the reflective type in the switched state, and in that the optical valve is formed of a cell having an absorbent state in the switched state.
As a result of these features, the reflection of the incident light penetrating the display assembly according to the invention no longer occurs on a reflective polariser placed behind the optical valve as was the case in the prior art, but directly on the display cell when the latter is switched into the second state in which it displays a data item. This thus allows any parasitic projection of the switched zones onto the reflective polariser to be eliminated, and thus prevents double display of these zones to the observer.
Moreover, unlike the prior art wherein the data was displayed in a dark colour on a totally reflective background, only the switched segments of the display cell are reflective and are displayed in a light colour on a dark background formed by the optical valve which is absorbent in the second switching state. Such a structure is thus less dazzling for the observer. It also allows the luminosity of the displayed data to be increased, and thus its legibility.
Finally, since the optical valve is absorbent in the switched state, it is no longer possible to see the bottom display device through it, from whatever angle one looks at the display assembly.
According to an advantageous embodiment, the display device and optical valve are in a transparent state in the absence of any voltage applied by the control means. Thus, the data displayed by the bottom display device is permanently visible, without the top display device consuming any power. This is particularly advantageous within the scope of an application to a portable object such as a wristwatch.
According to another feature of the invention, the display cell is formed of a twisted nematic type cell, an absorbent polariser placed in front of the cell, and a reflective polariser placed behind the cell.
According to yet another feature of the invention, the optical valve is formed of a liquid crystal cell of the twisted nematic type and an absorbent polariser placed behind the cell.